This tutorial will demonstrate the modelling configuration to simulate a power dissipator in an open channel. The dissipator design is proposed on the Stormwater Drainage Manual from the Drainage Services Department of Hong Kong and OpenFOAM will be used for the simulation with the interFoam solver since two immiscible and isothermal fluids are involved (water and air). The main variable of interest on the dissipator simulation is flow velocity to assess the efficiency of the dissipator.

About the design

The energy dissipator was developed for storm events in which channels bring higher flow rates and water velocity increases continuously; on this extreme flow conditions it is necessary to dissipate total water energy in the channel. The main characteristic of this energy dissipation device is that it doesn't rely on water jumps, but rather on water impact to the barrier which hits the flow so that it does not pass water with too much velocity/energy and decreases damages downstream. On the storm drainage management at city scale, there is a great amount of problems on sewers due to storm flows which bring high shear stresses that causes erosion on channel beds.

Dissipator geometry proposed by the Drainage Services Department of Hong Kong

It should be noted that not only a single dissipator design is necessary. Field data should be taken to perform the simulation of different dissipator geometries to assess the feasibility of the those dissipators for a specific area.

Model configuration

For the simulation of water flow at high velocity on the dissipator the InterFoam solver of OpenFOAM was selected since it allows to observe the potential phase changes and also to properly visualize interfase air/water. Some main features of interFoam are that is a solver for 2 incompressible, isothermal and immiscible fluids based on a volume of the fluid approach. InterFoam uses a transport equation to determine the relative volume of the two phases, or fraction of phase in each computational cell per time step.

Tetrehedrical mesh done with Salome-Platform and imported to OpenFOAM

The case study was modelled with defined conditions based on simulation visualization and dissipator/water interaction; each channel dissipator has to be designed according to an hydrograph for extreme floods.

Boundary and initial conditions

All the boundary conditions used in this exercise are simple and come from the boundary conditions of the "damBreak" exercise on the OpenFOAM documentation. In order to assure adequate energy dissipation, the channel inlet has a velocity of 30 m/s and flow is parallel to the x axis. On the model setup, the transport properties both fluids (air and water) were defined as Newtonian, any turbulence schema was not applied on this model to shorten the computational time,. Further changes on the flow conditions from laminar to turbulent can be made in the "turbulenceProperties" according to the needs or characteristics of the fluid. The barrier was conceptualized as separated in order to analyse it in more detail.

Flow simulation

To balance the Courant number and decrease problems with the InterFoAM solver, the time step of each iteration was reduced to "0.001" seconds, which will make our simulation longer, but it will decrease the potential of computational errors. The total simulation time is 40 seconds and last for more than 2 hours in a Corei7 desktop, in order to decrease the computation time it is recommended to run OpenFOAM on a Linux operating system with parallel processing.

Water velocity distribution before and after the dissipator

In the simulation schemes, all the derivative equations were maintained by default, the same happens in the solutions file, which has by default all the equations, tolerances and controlled algorithms.

Animations

These are two animations of the flow simualtion:

The first is the dissipator geometry and flow against the barrier.
The second is the velocity distribution on the bottom part of the dissipator

Real world application

As mentioned above, this modelling exercise is demonstrative and has the objective to show the modeling procedure, main input and output visualization of a barrier for the dissipation of kinetic energy and its transformation. For further analysis or for specific cases of stormwater drainage there is a need to analize several geometries and assess many extreme velocities related to different return periods to come up with some optimal configuration of the dissipator. Turbulence has to be taken into account for the OpenFOAM simulation, laboratory test with scale physical models are recommended to have a better analisys of the model parameter uncertainties.